This invention relates to an electric thruster and, more particularly, to controlling the temperature of the components of the electric thruster.
Electric thrusters are used in spacecraft such as communications satellites for stationkeeping and other functions. They may also be used for primary propulsion in deep-space and interplanetary missions. An important advantage of the electric thruster over an engine using chemical propellants is that it utilizes the electrical power generated by the solar cells or other power sources of the spacecraft to accomplish the propulsion. The electric thruster has a high specific impulse, making it an efficient engine which requires very little propellant. Since the electric thruster requires relatively small amounts of the consumable propellant, it is not necessary to lift large masses of propellant to orbit.
In an electric thruster, a plasma is created by electron bombardment of atoms and is maintained within the body of the thruster by a magnetic structure. Ions from the plasma are electrostatically accelerated rearwardly by an ion-optics system. The opposite reaction with the spacecraft drives it forwardly, in the opposite direction. The force produced by the electric thruster is relatively small compared with a chemical-propellant engine. The electric thruster is therefore operated for a relatively long period of time to impart the required momentum change to the heavy spacecraft. For some missions the electric thruster must be operable and reliable for thousands of hours of operation, through multiple starts and stops, and in throttling procedures where the power output of the electric thruster is adjusted as needed.
Most electric thrusters for spacecraft to date have been of relatively low power density. Current spacecraft plans contemplate much more powerful electric thrusters. Designs are needed for such higher-power electric thrusters. The present invention fulfills this need in part, and further provides related advantages.
The present invention provides an electric thruster that is suited for applications requiring increased power output and/or high-rate transient operations of the thruster. The inventors have recognized that a key limiting consideration for higher-power electric thrusters is removing the larger amount of by-product heat that is generated in the higher-power electric thruster. If the heat is not removed, the temperatures of the magnets of the magnetic structure rise above their temperature limits, so that the magnets lose their field strength and may become ineffective. Wiring and insulators may also be damaged. Excessively high temperatures may also warp the structure of the electric thruster and lead to structural failures.
Many of the structural shapes and materials of construction of the electric thruster are dictated by considerations of efficient creation of the plasma and ion extraction from it. It is also important to maintain the electric thruster as small in volume and as light in weight as possible. The ability to achieve high heat removal by reconfiguring the structural elements or by the selection of different materials of construction is therefore somewhat constrained.
The present invention utilizes a different approach. The surfaces of elements of the electric thruster are altered to increase their thermal absorptances to maximize heat absorption into these elements through their interiorly facing surfaces, and/or to increase their emissivities to maximize the radiation of heat from their exteriorly facing surfaces, and/or to increase the surface area through which heat is absorbed or emitted. The configuration of the elements and their base materials of construction are not altered. The electric thruster may be structurally optimized for performance, while at the same time achieving increased heat removal to allow the electric thruster to operate at higher power levels.
In accordance with the invention, an electric thruster comprises a housing having a wall with an opening therethrough. At least a portion of the wall of the housing has a surface treatment of a treated portion of its surface to increase a thermal transmission therethrough. The electric thruster further includes a source of a plasma within the housing, the plasma comprising electrons and ions of a propellant gas species, and an accelerator operable to extract the ions from the plasma and to accelerate the extracted ions out of the housing through the opening. The surface treatment is selected to increase the absorption of heat at the interiorly facing surfaces of the treated component, for example by increasing the thermal absorption coefficient (xcex1) of the interiorly facing surfaces and/or the surface area of the interiorly facing surfaces through which heat is absorbed, and/or to increase the radiation heat loss at the exteriorly facing surfaces of the treated component, for example by increasing the thermal emissivity coefficient (xcex5) of the exteriorly facing surfaces and/or the surface area of the exteriorly facing surface from which heat is emitted. These surface treatments have the effect of increasing the rate of heat transmission through the wall of the housing and keeping the interior cooler and/or allowing higher power densities to be used.
In a preferred form, an electric thruster comprises a housing that includes a lateral wall having a side wall and an anode wall disposed interiorly of the side wall. Optionally, a plasma screen is part of the lateral wall and is disposed exteriorly of the side wall. The housing further includes a back wall affixed to the lateral wall at a first end thereof. The back wall and the anode wall define a discharge chamber. A support structure is affixed to the lateral wall and to the back wall. At least one of the side wall, the anode wall, the plasma screen, the back wall, and the support structure has a surface treatment of at least a portion thereof to increase a thermal transmission therethrough. The surface treatments are as described previously and as will be described in more detail below. A magnetic structure is disposed within the housing and adjacent to the discharge chamber. A cathode assembly extends into the discharge chamber through at least one of the lateral wall and the back wall, a propellant gas inlet extends into the discharge chamber through at least one of the lateral wall and the back wall, and an ion-optics accelerator is affixed to a second end of the lateral wall.
A method for manufacturing an electric thruster comprises the steps of furnishing a set of the component elements of an electric thruster housing, and surface treating at least a portion of a surface of at least one of the component elements of the housing to increase the thermal transmission thereof. The method further includes furnishing an electron source, an ionization chamber, a propellant gas source, a magnetic structure, and an accelerator, and assembling the component elements of the housing, the electron source, the ionization chamber, the propellant gas source, the magnetic structure, and the accelerator together to form the electric thruster.
Some examples of operable surface treatments include anodizing the surface, roughening the surface, and applying a high-emissivity coating to the exteriorly facing surface. Coating procedures may include, for example, chromelizing the surface, black anodizing the surface, and depositing black nickel on the surface.
The present approach has the advantage that the essential functionality, configuration, and design of the thruster housing are not changed. The materials of construction and the configuration of the components may be selected and optimized for the operation of the electric thruster. Separately, the surface thermal properties of these components are modified to improve the removal of heat from the housing of the electric thruster. The electric thruster is therefore able to operate to higher power levels and/or with greater transient heat loadings than possible in the absence of the present approach.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.